1. Technical Field
The present invention relates to a circuit board, and more particularly to a multilayer circuit board.
2. Description of Related Art
FIG. 1 shows a conventional multilayer circuit board 1, including a plurality of substrates 10 with several plated through holes 12. Electronic components (not shown) are provided on exterior surfaces of the top and the bottom substrates. The top substrate 10 is provided with a plurality of pads 102 for a specific device, such as wires or probes (not shown), to conduct an electrical connection. Each substrate 10 has a sub-circuit 104, and the plated through holes 12 conduct electrical connections of the sub-circuits 104 form a circuit.
The substrates 10 have to be precisely positioned in the lamination process for aligning the through holes 12. However, the precise alignment is harder when there are more substrates in the multilayer circuit board. Therefore, the yield rate of making the multilayer circuit boards is low. In addition, the signal transmission in the plated through holes 12 might be interfered while there are traces (shown by dot lines in FIG. 1) around the through holes 12. Typically, the plated through hole 12 includes an effective section 12a, which is between the sub-circuit and the pad for signal transmission between the sub-circuit and the pad, and a null section 12b, which is under the sub-circuit and transmits no signal. The effective sections 12a of the plated through hole 12 will be longer while there are more substrates, and the alignment problem will be more significant. However, there usually are parasitic capacitances and parasitic inductances in the null sections 12b. So, the signal interference problem will be more serious while null sections 12b are longer.
Furthermore, the pads 102 usually are provided on the same surface of the multilayer circuit board 1. It is easy to understand that the pads 102 take a space of the surface, and there will be less space for the traces while there are more pads 102 on the surface, and that might be a problem of insufficient traces to connect to all the pads 102.